1. Field of the Invention
The present invention relates to a microphone and a method of manufacturing a microphone.
2. Description of the Related Art
Microphones, which receive acoustic waves that propagate from a sound source and identify the direction along which the acoustic waves propagate, have been developed. The direction along which the acoustic waves propagate may be referred to hereinafter as the direction of the sound source. When the direction of the sound source can be identified, only the acoustic waves propagating from the sound source can be received, thus a microphone having directional characteristics can be realized. The technology regarding a microphone that identifies the direction of the sound source is disclosed in a publication titled “Design and Experiments of Bio-mimicry Sound Source Localization Sensor with Gimbal-Supported Circular Diaphragm”, authored by Nobutaka ONO, Akihito SAITO, and Shigeru ANDO, published in the Proceeding of The 12th International Conference on Solid-State Sensors, Actuators and Microsystems, Boston Jun. 8-12, 2003, pp. 939-942.
Note that the word “microphone” in the present specification not only means a device that receives sound and converts that sound to electrical signals, but also a general concept that includes a device that identifies the direction of the sound source.
In the technology disclosed in the above publication, four electrodes are arranged on the rear surface (the surface opposite the surface which receives acoustic waves) of a diaphragm that is supported at the center portion thereof. The four electrodes are arranged at substantially equal intervals around the center portion of the diaphragm. Four other electrodes are arranged facing these four electrodes respectively. A gap of predetermined length is formed between each electrode arranged on the rear surface of the diaphragm and each electrode facing thereto. A voltage is applied between each electrode on the diaphragm and each electrode facing thereto. Thus, capacitors are formed by each electrode on the diaphragm and each electrode facing thereto. When the diaphragm vibrates, the length of the gap between each electrode on the diaphragm and each electrode facing thereto will change. The capacitance of the capacitor will change in response to the change in gap length.
When the microphone receives acoustic waves propagating from a certain direction, the diaphragm will vibrate. Because the diaphragm is supported at the center portion thereof, the periphery of the supported center portion will vibrate. The vibrations produced around the periphery of the diaphragm may not be uniform, and thus there will be regions distributed around the diaphragm in which the amplitude of the vibration is large, and other regions thereon in which the amplitude of the vibration is small. This distribution depends upon the direction of the sound source. On the other hand, the vibrations cause a change in the gap length between each electrode on the diaphragm and each electrode facing thereto. Thus, the distribution of the amount of change in the gap length will change depending upon the direction of the sound source. In other words, the distribution of the amount of fluctuation in the capacitance of each capacitor will also change depending upon the direction of the sound source. Thus, the direction of the sound source can be identified from the distribution of the amount of fluctuation in the capacitances of the capacitors.